Root canal treatment becomes necessary when the tooth gets rotten or the pulp dies due to any other causes.
During the treatment, the dentist opens the root canals in the particular tooth, cleans them, sterilizes and closes them with root filling preventing the bacteria from getting in the tissues. In case the tooth is already inflamed, temporary sensitivity is natural.
In case of teeth with complicated anatomy, it might happen that not all the canals and branches can be opened up or provide filling all along the canal. If so, some canals or part of canals may still have bacteria or self-produced toxins in them preventing the inflammation from going away.
Dead teeth, after root canal treatment and root filling, can go dry and become fragile which often leads to the removal of the tooth. That is why, especially in case of back teeth, crown is advisable to be fitted on the root canal treated tooth.